Optical modulator, method for fabricating the same, and photonic semiconductor device

A photonic semiconductor device comprising:

• an optical modulator having;
• a semi-insulating semiconductor substrate with a principal plane partially including an exposed surface,
• an optical waveguide ridge which is disposed on said semiconductor substrate and which includes a first clad layer of a first conductivity type, an optical-absorption layer, and a second clad layer of a second conductivity type, said optical waveguide ridge further having a side with a flat portion extending uniformly from a top of the ridge to said semiconductor substrate, the flat portion being in contact with the exposed surface of said semiconductor substrate,
• a dielectric film which covers said optical waveguide ridge and said semiconductor substrate and which has a first opening made at the top of said optical waveguide ridge and a second opening made in a region of said semiconductor substrate other than the exposed surface,
• a first electrode disposed on said dielectric film and mounted through said first opening on the top of said optical waveguide ridge, said first electrode further extending on the flat portion of said optical waveguide ridge while in close contact with a surface of said dielectric film, said first electrode further having one end thereof established on said semiconductor substrate through the exposed surface thereof, and
• a second electrode disposed on said semiconductor substrate and connected to the first clad layer through the second opening of said dielectric;
• and
• a semiconductor laser device aligned in optical axis with the optical absorption layer of said optical modulator.

An optical modulator fabricating method including the steps of:

• forming firstly a first clad layer of a first conductivity type, an optical absorption layer, and a second clad layer of a second conductivity type on a semi-insulating semiconductor substrate;
• forming secondly by photolithograpy and etching an exposed surface of the semiconductor substrate as well as an optical waveguide ridge which has a side with a flat portion stretching uniformly from a top of the ridge to the semiconductor substrate, the flat portion being brought into contact with the exposed surface of the semiconductor substrate;
• forming thirdly a dielectric film over the semiconductor substrate and a first and a second opening through the film, the first opening being made at the top of the optical waveguide ridge, the second opening being made in a region of the semiconductor substrate excluding the exposed surface thereof;
• forming fourthly a first electrode through the first opening on the top of the optical waveguide ridge in such a manner that the first electrode extends on the flat portion of the optical waveguide ridge while in close contact with a surface of the dielectric film, the first electrode further having one end thereof formed on the semiconductor substrate through the exposed surface thereof;
• and
• forming fifthly a second electrode connected to the first clad layer through the second opening of the dielectric film.